Compositional dependence of the optical parameters of GexSb40-xSe60 glasses

The optical bandgap (E-g) and the complex index of refraction ((N) over til de) in GexSb40-xSe60 glasses are studied by means of spectroscopic ellipsom etry in the light wavelength region of 400 to 820 nm. The values of optical bandgap, E-g, and complex index of refraction, (N) over tilde, are conside red as a function of the average coordination number (Z) of covalent bonds per atom. Their compositional dependences show changes in the vicinity of Z =2.67, which we associate with the topological phase transition, occurring when the predominantly two-dimensional layer structure (2D) changes to a th ree-dimensional (3D) network. (C) 1998 Elsevier Science B.V. All rights res erved.